Recently, interest in energy storage technologies have increased. As the energy storage technologies are extended to such devices as cellular phones, camcorders and notebook PCs, and further to electric vehicles, the demand for a high energy density battery used as a power source of such an electronic device has increased. A lithium ion secondary battery is one of the most satisfactory batteries, and numerous studies towards improvements are now in progress actively.
A lithium secondary battery may be made in various shapes. Representative examples include an angled lithium secondary battery, a cylindrical lithium secondary battery and a pouch-type lithium secondary battery.
As is well known in the art, a cylindrical lithium secondary battery generally includes an anode made of carbon materials and a cathode made of lithium oxide, wherein the anode is capable of occluding and emitting lithium ions, a separator bridging the anode and the cathode and preventing the anode and the cathode from coming into electrical contact and a non-aqueous electrolyte. For example, such a cylindrical lithium secondary battery is manufactured by mounting a jelly-roll type electrode assembly having a cathode, an anode and a separator into a metal can, welding the anode of the electrode assembly to a lower end of the can, then injecting a non-aqueous electrolyte into the can, and finally welding the cathode of the electrode assembly to a protruded terminal of a top cap so as to seal the battery.
Such a cylindrical lithium secondary battery has a large capacity, so it should be treated safely and carefully. To this end, a CID (Current Interrupt Device) is applied to the cylindrical lithium secondary battery so as to interrupt the electric current and lower the inner pressure when the battery operates abnormally.
FIGS. 1a to 1c show a conventional operation process of the CID subsequently.
Referring to FIGS. 1a to 1c, a top cap 10 has a cathode terminal in a protruded shape, and an exhaust hole is perforated therein. A PTC (Positive Temperature Coefficient) element 20 is provided below the top cap 10 to intercept an electric current by increasing a battery resistance greatly when a temperature inside the battery is increased. A safety belt 30 is protruded downward in a normal state, but it is protruded upward and ruptured to exhaust gas when an inner pressure of the battery is increased. A connection plate 50 is coupled with the safety belt 30 at one upper side and connected to the cathode of an electrode assembly 40 at one lower side. The top cap 10, the PTC element 20, the safety belt 30 and the connection plate 50 are positioned in order.
Thus, under normal operation condition, the cathode of the electrode assembly 40 is electrically connected to the top cap 10 via a lead 42, the connection plate 50, the safety belt 30 and the PTC element 20.
However, if gas is generated from the electrode assembly 40 and an inner pressure is increased due to a factor such as overcharging, the safety belt 30 inverts its shape to protrude upward as shown in FIG. 1b, and at this time the safety belt 30 is separated from the connection plate 50 to interrupt the electric current so that any overcharging is not continued and the safety of the battery is secured. Nevertheless, if the inner pressure keeps increased, the safety belt 30 is ruptured as shown in FIG. 1c, and the pressurized gas is exhausted through the exhaust hole of the top cap 10 via the ruptured portion, thereby preventing explosion of the battery.
Meanwhile, a device such as notebook to which a cylindrical battery is applied may be exposed to various environments. For example, a user may leave such a device at a high temperature for a long time in a fully-charged state. In this case, though the safety of the battery is ensured, a CID may be activated at an early state due to a temporary increase of inner pressure.
In order to solve this problem, there has been suggested a method of controlling the amount of electrolyte injected into the cylindrical secondary battery or controlling the CID circuit breaker. However, this method deteriorates the safety of the battery. That is to say, it is not easy to solve the safety problems of the battery both regarding over-charging and exposure to a high-temperature environment.